There are many applications where an object must be positioned with high precision in the nanometer range. Such applications include scanning tunneling microscopy, precision mechanically suspended linear slides and XY stages, such as those used in the manufacture of electronic circuit chips, various precision optical applications, and diamond turning machines. In such applications and others, the object must not only be precisely positioned, but the position must be maintained regardless of perturbations to the object resulting from air currents, vibration, temperature variations and the like. In some applications, such a fine position control mechanism may be used alone, while in other applications it may be used as a fine motion control stage in a mechanism which also includes a coarse motion control stage for positioning the device over larger distances with a lower degree of resolution.
While various bearings have been utilized in the past in such applications, including mechanical bearings, flexures, gas and liquid fluid bearings, etc., magnetic bearings have been found preferable for such applications for a number of reasons. Such bearings or suspensions have superior controllable stiffness, have less cross-coupling between modes with multiple degrees of freedom and are generally easier to control. They are not subject to rough spots, friction or wear as with mechanical bearings, and are also simpler to design mechanically, generally involving only a single moving part. However, while magnetic bearings offer a number of advantages, they also provide various control problems, particularly when operating with nanometer or Angstrom resolutions.
One approach to developing a magnetic bearing for fine motion control is discussed in an article entitled "Design Considerations for Ultra Precision Magnetic Bearing Supported Slides" by A. H. Slocum and D. B. Eisenhaure, NASA Magnetic Suspension Technology Conference, Hampton, VA Feb. 2-4, 1988. However, the device described in this paper had a number of limitations which adversely affect its performance.
First, while permanent magnets were used in this device to support the weight of the object or platen to be moved, the gap for the permanent magnet was substantially the same as the gaps for the electromagnets used for positioning the platen or maintaining the position thereof. These gaps were relatively small, in part because position sensing capacitive probes were mounted within the coils of the electromagnets. This results in a relatively large unstable frequency of suspension, thereby making the control problem far more difficult. One of the objects of this invention is to provide enhanced stability in a small motion system.
Another problem with the prior system is that it did not operate the electromagnets in push/pull mode in all dimensions and for all degrees of freedom. This inhibited the ability to correct for various errors and also reduced stability of the system. The control problem was also complicated by using magnet of different size. Other problems included mounting the probes in the electromagnets which increased the size, and thus the required currents, for the electromagnets and also restricted the gaps available for these magnets. Position sensing was also restricted to less than the full range of motion of the platen, resulting in instability when the platen moved outside the sensor range. Further, the platen was a hollow tube which exhibited significant resonance at one or more selected frequencies. It is desirable that the platen not be designed to permit such resonance.
The prior art system also was unable to compensate with the permanent magnet for variations in platen weight which might occur, for example, when a load was placed on the platen to have work performed thereon. Since enhanced stability and control is achieved, and electrical usage is minimized, where the platen weight is always being supported solely by the permanent magnet, regardless of changes in the weight of the platen, a need exists for a mechanism to permit the magnetic force of the permanent magnet exerted on the platen to be adjusted to achieve this objective.
In addition, it is desirable in some applications that the system be damped beyond the damping provided solely by the magnetic bearings. Since air provides little damping, such additional damping could be provided by suspending the platen in a viscous fluid, ferrofluid, or similar media for improving damping and high frequency coupling between the platen and a frame or housing in which the platen is mounted. Where such fluid is provided, it might also be utilized to carry the weight of the platen in lieu of or in addition to the permanent magnet and to adjust for weight changes of the platen. Where a ferrofluid is employed, it may also be utilized, in conjunction with the electromagnets, to control platen or object position in some or all degrees of freedom.